The main intrinsic factors are pulp hemorrhage, decomposition of pulp, bacteria and their products, tetracycline, pulp necrosis, intracanal medicaments, some endodontic filling materials, and metallic restorations, With the dissemination of blood components into the dentinal tubules caused by pulp extirpation or traumatically induced internal pulp bleeding, blood vessels are broken, promoting blood overflow into the pulp chamber.
Source: wiktionary